Smart antennas, or adaptive antenna arrays are proving to have a distinct advantage in modern wireless communication systems. The array is capable of beamforming or directing a beam of radiated energy toward a desired receiver. This has the advantage of increasing the power available to the desired receiver. In code division multiple access (CDMA) systems this has proven even more advantageous because the interference received by another receiver in an adjacent area is significantly reduced relative to conventional omni-directional or sectored transmissions.
The beamforming process is simplified in a time division duplex (TDD) system because the communications to and from an adaptive antenna array occur on a common frequency albeit during different time slots. This enables the adaptive antenna to create a transmit beam pattern substantially equal to a beam pattern received by the antenna array. As opposed to a frequency division duplex (FDD) system where transmissions and receptions occur on different frequencies, having different propagation paths, the beamform of a TDD system is more accurately formed because the transmit and receive propagation characteristics are substantially the same.
An adaptive antenna array uses a multitude of transmitters and receivers, each coupled to a corresponding element of the antenna array. The received beamform is determined by measuring the power and phase of a desired signal received on each antenna element and its corresponding receiver. The transmit beam is formed by varying the phase and power of each signal transmitted by the multitude of transmitters and corresponding antenna elements.
In order to accurately form a desired beam, the amplitude and phase of each component of the adaptive array subsystem should be known to a reasonable degree of precision. Uncompensated differences in the gain and phase in the adaptive antenna array subsystem degrades the performance of the subsystem. Ideally the gain and phase characteristics are predetermined at the time of manufacture and are environmentally invariant. However, in reality these characteristics vary over time, in response to the environment and in response to servicing and replacement of components of the adaptive antenna array subsystem. Thus, what is needed is an apparatus for determining variations in gain and phase of components of an adaptive array subsystem and for producing a calibration signal in response thereto.